The first thing to do before adding a second battery is to
decide if you really need one. One thing you must consider is that a car battery only
provides power to run electrical devices when the car is off. When the car is running, the
battery is actually drawing power from the electrical system, away from other devices. So,
if you want to be able to play your system for long periods, and still start your car, a
second battery may be for you, but there are still a few alternatives.

Many of you are looking at competitions, where you will be sitting
for long periods, showing your car. Of course, with your doors open, all of your interior
lights are on, plus the amps, processors, heads, accessory lights, motors, actuators, and
so forth. One alternative is a high current inverter. One of these can easily be made from
a wind up drop cord and a battery charger. All you have to do is be within 30 feet of a
plug at a show, and you can plug your car in and avoid killing your one battery.

Another alternative is the Priority Start, or a similar device.
The Priority Start from BLI International is about 80 dollars (which is less than a new
good quality battery and add on kit), and it goes between your battery cable and your
battery. Priority Start will monitor your system voltage. If the battery charge gets
dangerously low, it will disconnect the battery. You can then hit the reset switch, and
start your car to recharge your battery. A special note is that you should have a backup
battery for your alarm, radio, and clock, because when the battery is disconnected, your
memory is lost.

Now, if after everything, you still want a second battery, then do
it. First, you have to take special considerations. For starters, you have to find a
suitable mounting location. Most cars nowadays do not have enough room under the hood as
it is, much less for a second battery. If you can find, or make room under your hood, good
deal. If not,we have to look at the trunk. If you don't have a trunk, then you have a
problem. As a battery discharges, it gives off gas (normally hydrogen, but under certain
conditions, sulfur dioxide), and even the best sealed batteries leak gas (much like Eddie,
who despite being a tightass, leaks gas). Also, if you have an accident, there is a
possibility of sulfuric acid going everywhere.

If you have a hatchback, you should be prepared to build a
waterproof compartment for your second battery,and vent it to the outside. The easiest
thing to do is build a small box out of wood or MDF, and then use fiberglass resin to seal
it in. Use RT. Silicone to make a gasket for the lid. Have one side vented to the outside.
Normally, most cars have a vent on either side panel in the back. Affix a fan to the vent
so that it evacuates air from the battery compartment.

Now, for securing the battery. Most autoparts stores will have a
battery tray and mounting hardware. You can screw the tray to the floor of your car, then
bolt the battery in. It is essential to secure your battery. In an accident, an insecure
battery can fly around, cracking the case, and leaking acid. It can also be a fire hazard.
An insecure battery can easily tip in a turn, shorting out. A battery tray is normally
less than 20 dollars. A new trunk can be a few grand.

Now that your battery is mounted and secure, you need to connect
your second battery to the main battery. Normally, you can just connect the battery right
up, but you may want to consider a battery isolator. A battery isolator will allow both
batteries to charge, but if one battery should go dead, the second battery will be
unaffected. Then, using a good battery switch, you can switch to your second battery, or
even have both batteries connected at the same time for more power (argh argh argh!).

Because
some people who read this may have a match between their shoe size and their IQ, I'd like
to point out that your battery switch is not just a normal switch or relay. A battery will
put out several hundred amps of current at a time. Because of this, special high current
switches should be used to handle these high loads. These switches can be obtained at an
autoparts store also

With your battery in the back, you will need a very
large wire to connect it to the front. Because the second battery could be called upon to
start the vehicle, you should get the biggest cable you can. 1/0 wire is suitable for this
application. Fuse your batteries near the + terminals, then run your wire (under the car
is probably the best way for this unusually large wire). Up front, you want to connect the
wire to your isolator, then to the primary battery. Then, you'll wire up your switch so
that it bypasses the isolator. Instructions for wiring up the switch will be included in
the package.